This invention relates to certain liquid crystal compounds of the smectic type. More particularly, it relates. to smectic liquid crystals which exhibit a ferroelectric, chiral smectic phase and which find application in electrooptic display devices.
The utilization of the properties of a ferroelectric smectic phase to effect a switching phenomenon in an electrooptic display device has been known and is described by N. A. Clark and S. T. Lagerwall in Appl. Phys. Lett. 36, 899 (1980). Such devices operate with low electric power consumption while providing a more rapid switching than is realized in electrooptic display devices dependent upon the properties of liquid crystals of the nematic type. Display devices of the ferroelectric type rely upon two tilted configurations of smectic liquid crystal molecules to provide two states of equal energy. Switching from one state to the other is accomplished by moving a boundary between the two domains by applying an electric field across a pair of electrodes sandwiching a layer of the ferroelectric liquid crystal material.
Examples of ferroelectric liquid crystal compounds of the smectic type are described, for example, by G. Decobert and J. C. Dubois in Mol. Crystl. Liq. Crystl., 1984, 114, 237-247; by J. W. Goodby and T. M. Leslie in "Liquid Crystals and Ordered Fluids", Edited by A. C. Griffin and J. F. Johnson, Plenum Press, Vol. 4, pp. -32; and in U.S. Pat. No. 4,576,732 (issued Mar. 18, 1986 to M. Isogai et al.). Typically, a ferroelectric liquid crystal phase will be a chiral smectic pase such as a chiral smectic C phase (S.sub.c *), a chiral smectic H phase (S.sub.H *), a smectic F phase (S.sub.F *), or a chiral smectic I phase (S.sub.I *). These chiral smectic phaes are characterized in tht the molecules are oriented and form layers. The molecules are aligned aslant in a specified direction on the surface of the layers. The direction of the slope is shifted little by little between the layers so that the phase, as a whole, has a helical structure (R. B. Meyer, Mol. Cryst. Liq. Cryst., 40, 33, 1977).
In the aforeciated U.S. Pat. No. 4,576,732, it is indicated that the appearance of ferroelectricity in a molecular structure is dependent upon two conditions--the presence of an optically active group and the presence of an electric dipole in a direction approximately perpendicular to the major axis of the liquid crystal molecule, to induce spontaneous polarization. In general, it is well recognized that the suitability and operating efficiency of an electrooptic device will be dependent upon the chemical structure of the liquid crystal compound employed therein and that the properties of the liquid crystal compound will be influenced by molecular factors such as rigidity or stiffness, morphology, crystallinity and intermolecular forces.
In the production of ferroelectric liquid crystal compounds of the smectic type for application in a ferroelectric device, it will be a fundamental requirement that the liquid crystal compound (or a liquid crystalline mixture of compounds) exhibit chemical stability. Moreover, it will be highly desirable if the liquid crystal material shows a smectic phase over a wide range of temperatures, particularly, at temperatures in the region of room temperature. In addition, and ideally, the liquid crystal material will exhibit a high level of spontaneous polarization (P.sub.S) in order to optimize coupling to an applied electric field. Accordingly, there is considerable interest in chemically stable liquid crystal compounds and compositions which show desirable spontaneous polarization values and smectic behavior over a wide range of temperatures and which can be employed in an electrooptic device for high-speed optical switching, particularly at room temperature.